Valley Fever on the Rise in U.S. Southwest, with Links to Climate Change

Dry dust baked in this summer's heat wave might be driving up valley fever cases across the southwestern United States.

California and Arizona have reported increases in this fungal infection this year, part of a rising tide spanning the past decade. "Recent increases in rates have been reported," said Matt Conens at the California Department of Public Health in an email. "The reasons are unclear but may include changes in reporting, cyclical changes and increased migration into endemic areas." Some experts, however, think the changing climate may also be an ingredient.

The illness is caused by Coccidioides immitis and Coccidioides posadasii, fungi that dwell in certain types of soil found mainly in desert regions spanning northern Mexico, Texas, Arizona, New Mexico and Southern California.

Antje Lauer, a microbiologist at California State University, Bakersfield, explained that in most environments, bacteria and other organisms keep the pathogen in check. But when the temperature rises and rainfall slows to a trickle, these hurdles fall and the fungi can surge. Lauer said in these conditions, Coccidioides forms spores to survive and give it a head start once the rain starts to fall again.

These spores can break off in the desert wind, lofted to areas where people can breathe them in and causing coccidioidomycosis, also known as valley fever. "Only a few spores are needed to cause the disease," Lauer said.

Roughly 60 percent of infected people do not experience any problems, but the remainder can suffer flu-like symptoms along with rashes and muscle pain. In a small fraction of cases, the fungus can spread beyond the lungs into other tissues, which can be deadly.

In the United States, about 150,000 fall ill from coccidioidomycosis annually. Between January and July this year, 8,969 valley fever cases were reported to the state, almost double the five-year median of 4,507, according to the Arizona Department of Health Services. Conens said data on this year's illnesses in California won't be available until next year.

Difficult to monitor, easy to misdiagnose
However, researchers said these numbers might not reflect reality, since the disease is frustratingly mercurial, as it is difficult to monitor, diagnose and treat.

"We're only capturing the tip of the iceberg," said Clarisse Tsang, acting manager of the Infectious Disease Epidemiology Program at the Arizona Department of Health Services. She observed that many people who do get sick don't end up going to the doctor, so those cases are not reported. Of patients who do get medical attention, many physicians misdiagnose the illness. Doctors who get it right do not always let state health officials know.

The disease also takes from one to four weeks to develop, so there is a substantial lag between when people are exposed and when state health officials start finding out.

As a result, health officials only have a loose grasp on how valley fever is changing and spreading. This has tremendous consequences for employers, since infected individuals miss a median of 14 days of work, according to Tsang. "The economic impact of valley fever is probably grossly underestimated," said Joe Tabor, an assistant professor at the College of Public Health at the University of Arizona.

Figuring how valley fever interacts with the environment has also been a struggle. Unlike viruses or bacteria, Coccidioides cannot spread between people and is not transmitted through vectors like mice and mosquitoes; it is simply from the environment. "Most of the understanding of the ecology of valley fever occurred in the '60s, and not a lot of progress has been made," Tabor said.

Hard to capture
Part of the reason is data are hard to come by. Coccidioides spores are difficult to find in the soil, growing in patches spread out over wide areas. "You can move over just a few feet and you can't find it," Tabor said. Tracking the disease in the air is even more troublesome, he added, noting that no one has yet isolated spores from air samples, although one attempt to do so led to a scientist coming down with valley fever.

"The only things we have to go by are the actual health data," said James Tamerius, a postdoctoral researcher in environmental health sciences at the Mailman School of Public Health at Columbia University. Based on this information, he noted, some patterns have started to emerge.

Tamerius explained that heavy fall and winter precipitation preceded previous spikes in valley fever by a year or so. He also noted that the disease tends to cluster near swaths of desert. "These big, open environments are what's necessary," he said. Cases also emerge as the dust is perturbed, often on construction sites and sometimes from dust storms that occasionally blanket the region.

As for the climate, Tamerius said it is hard to tell if warming temperatures are having an effect on the increases in the disease. "Some people think it has to do with changes in climate. Some people think it has to do with people moving in from out of state," he said, noting that previously unexposed people, like retirees settling down in Phoenix, may be at greater risk. The Centers for Disease Control and Prevention notes on its valley fever website that 30 to 60 percent of people who live in endemic regions are exposed to the fungi at some point in their lives. Better surveillance, awareness and reporting may also be pushing numbers higher.

Researchers are concerned because there is no reliable way to anticipate a surge or outbreak, or even increased risk for the disease. "There's not enough ecological data at the sub-meter scale to make those predictions," Tabor said. Valley fever's regional impacts also make it harder for local officials to garner national attention and research support.

For now, health officials are pinning their hopes on a vaccine to push back against dust clouds of infection. "This is a disease that's very hard to prevent because you can't tell people not to breathe," Tsang said.